The present invention relates to a composition for preventing or treating fatty liver or insulin resistance syndrome comprising an extracellular domain of DLK1 (delta-like 1 homolog) as an active ingredient.
Fatty liver is the condition of abnormal fat accumulation in hepatocytes, and is medically the disease state caused when the content of triglyceride exceeds at least 5% of the total liver weight. In general, fatty liver is divided into two categories; alcoholic fatty liver (alcoholic fatty liver disease, ALD) caused by continual excessive drinking and non-alcoholic fatty liver (non-alcoholic fatty liver disease, NALFD) displaying similar liver tissue features to alcoholic fatty liver even though there is no alcohol drinking. According to high fat and high calorie diet in the modern society, which increases adult diseases, non-alcoholic fatty liver disease (NALFD) is noticed in 20˜30% of adult population in advanced countries, among which 2˜3% of the people progress to nonalcoholic steatohepatitis (NASH) with displaying steatohepatitis features accompanied with tissue fibrosis and inflammation that increases the risk of liver cirrhosis, liver failure, and liver cancer.
Insulin plays an important role in using blood glucose as an energy source. Insulin delivers glucose to each cell by using an insulin receptor on each cell membrane. Insulin resistance is a glucose/nutrition associated metabolism disorder, which is caused when the liver tissue, adipose tissue, and muscle tissue do not respond normally to a normal concentration of insulin. Insulin promotes glucose absorption in muscle or regulates blood sugar by suppressing glucose production in the liver. Insulin resistance indicates such state that the insulin activity is reduced under the normal insulin concentration (Schulman G I, J. Clin. Invest. 106:171-176(2000)). Insulin resistance has been known as a major cause of many dangerous factors causing such diseases as type II diabetes, obesity, hypertension, hypertriglyceridemia, low HDL cholesterolemia, coronary artery disease, and artherosclerosis.
Noch signal transduction has been well preserved in from vertebrates to invertebrates in the prospect of evolution, which allegedly plays an important role in determining the cell fate in the early developmental stage. Notch signal transduction is also known as a key factor in the regulation of differentiation of nerve, eyeballs, lymph, muscle, and blood corpuscles. Abnormal regulation of Notch signal transduction is directly involved in the development of various diseases including congenital diseases and cancer. Notch signal transduction has also been confirmed to be an important factor involved in non-alcoholic fatty liver disease and diabetes. Notch activation promotes glucose synthesis and fat generation in hepatocytes, and thereby increases insulin resistance (Pajvani U B et al., Nat Med 2011; 17: 961-967).
Mammals have 4 Notch receptors (Notch 1, 2, 3, and 4). Each Notch receptor is synthesized as a 300˜350 kDa protein and forms a heterodimer on the cell surface when S1 region is cut off by furin-like convertase in the Golgi body. 4 Notch ligands (Jagged-1/2 and Delta-like-1/3/4) were also identified in mammals.
DLK1 (delta-like protein 1) belonging to notch/delta/serrate family is a transmembrane glycoprotein encoded by dlk1 gene on chromosome 14q32, which is composed of 383 amino acids. This protein is composed of 280 extracellular domains, 24 transmembrane domains, and 56 intracellular domains. It has 6 epidermal growth factor like repeat domains outside of the cell membrane and 3 N-glycosylation sites and 7 O-glycosylation sites. DLK1 is a transmembrane protein and at the same time acts as an independent protein functioning by shedding when outer layer of the cell membrane is fallen apart from the cell membrane by tumor necrosis factor alpha converting enzyme (TACE) (Yuhui Wang and Hei Sook Sul, Molecular and cellular biology. 26(14): 5421-5435, 2006).
DLK1 (delta-like 1 homolog) is mainly expressed in the early developmental stage of embryonic tissue (Smas C M et al., Cell. 73: 725-34, 1993; Kaneta M et al., Journal of Immunology. 164: 256-64, 2000) and placenta. It is especially detected at a high level in maternal serum, because of which it has been known as fetal antigen 1 (FA1) (Jensen C H et al., European Journal of Biochemistry. 225: 83-92, 1994). According to the previous reports, DLK1 is also expressed in glandular cells, ovary, and skeletal myotubes, etc. DLK1 expression disappears in most tissues after birth and is only observed in such specific cells as preadipocytes, pancreatic islet cells (Carlsson C et al., Endocrinology. 138:3940-8, 1997), thymic stromal cells (Kaneta M et al., Journal of Immunology. 164: 256-64, 2000), and adrenal gland cells (Halder S K et al., Endocrinology. 139: 3316-28, 1998).
Numbers of research results support that DLK1 interacts with Notch and has the activity of inhibiting Notch signal transduction. The water-soluble extracellular domain of DLK1 produces TACE (tumor necrosis factor-α converting enzyme) protease, by which it can inhibit adipogenesis in vivo and in vitro (Wang Y et al., Mol Cell Biol 2006; 26: 5421-5435). Based on those research results, it is expected that DLK1 can be developed as a target material to regulate metabolic disorders observed in fatty liver, insulin resistance syndrome, type II diabetes, and non-alcoholic liver disease.
The present inventors tried to develop a novel agent to treat and improve fatty liver or insulin resistance syndrome. In the course of the study, the present inventors confirmed that a pharmaceutical composition comprising the DLK1-Fc fusion protein constructed by the conjugation of an extracellular domain of DLK1 or a fragment thereof with a human antibody Fc region could reduce triglyceride in the liver, improve glucose and insulin resistance, and inhibit fat accumulation and glucose synthesis in the liver, leading to the completion of the present invention.